The present invention relates generally to the production of shadow masks and, more particularly, to a method of assembling a plurality of shadow mask sections for fabricating a larger-sized shadow mask.
The use of shadow masks for precision patterning is a well-known art. Due to the ever-increasing need for higher resolution displays, greater shadow mask dimensional accuracy is desirable. One of biggest factors that adversely affects said dimensional accuracy is temperature changes due to thermal expansion. Temperature changes during mask manufacturing, as well as during the deposition process, result in shadow mask dimensional errors especially for larger-sized shadow masks.
In the manufacturing of organic light emitting diode (OLED) displays, shadow masks are used to pattern the electroluminescent coatings onto a substrate. Typically these shadow masks are fabricated using one of a number of different manufacturing processes. The two most common mask manufacturing methods are electroforming and etching. In order to reduce manufacturing costs of an OLED display, the displays are fabricated in a batch-type process in which many displays are fabricated on a single substrate and then separated with a scribing and breaking process. Generally speaking, the larger the substrate the lower the unit cost of the display. To pattern coatings on larger substrates a larger mask is used. Unfortunately current mask manufacturing processes do not permit fabrication of larger-sized shadow masks with the desired level of precision and quality.
The need for higher resolution displays also creates the need to manufacture masks with smaller coating apertures that permit smaller pixels sizes in an OLED display. Using either the electroforming or etching manufacturing processes, fabricating larger size masks with very small coating apertures becomes difficult due to the increased likelihood that the mask will possess quality defects. The present invention overcomes the dimensional inaccuracies and quality issues of a larger-sized shadow mask for the manufacturing of high-resolution displays.
Currently, fabrication of OLED displays is accomplished with a series of vapor deposition processes to apply thin coatings of material to a substrate such as a piece of glass. To create a pattern within the coating, a shadow mask is placed over the surface to be coated. To obtain the best pattern xe2x80x9cimagexe2x80x9d, the shadow mask is kept as thin as possible and is held in Intimate contact with the surface to be coated during the deposition process. One means used to facilitate contact between the shadow mask and substrate is to fabricate the shadow mask from a thin ferromagnetic material by means of electroforming or chemical etching and then place a permanent magnet in close proximity to the substrate side opposite the shadow mask. If movement of the shadow mask is not impeded in a direction normal to the substrate surfaces and the shadow mask possesses no permanent deformations (e.g. wrinkles), then the desired contact between the shadow mask and substrate is achieved.
Typically shadow masks are mounted to a rigid mask frame. The primary functions of the rigid mask frame are to (1) provide structural integrity needed to handle shadow masks either manually or with automated equipment without causing damage to the mask, (2) provide a means to handle and align the shadow mask to existing patterns on the substrate, and (3) provide a means to support the shadow mask with a limited amount of mask sag when held in a horizontal orientation (by bonding the perimeter of the mask to the rigid frame).
It is therefore an object of the present invention to provide a method for fabricating large shadow masks by precisely interconnecting a plurality of shadow mask sections to form a single shadow mask array.
It is a further object of the present invention to provide a method for fabricating large shadow masks which distributes error in individual mask sections across the entire mask assembly thereby achieving better accuracy across the mask section array than that which could be achieved from a full-sized one-piece mask.
Yet another object of the present invention is to provide a method of fabrication of a shadow mask that is dimensionally more precise and comprises fewer defects as compared to a shadow mask fabricated as a single larger sheet.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing a method for making a shadow mask array comprising the steps of producing a plurality of shadow mask sections; inspecting the plurality of shadow mask sections to determine which shadow mask sections are acceptable as meeting predetermined criteria; positioning a predetermined number of the acceptable mask sections into predetermined locations relative to one another to form a mask section array thereby distributing any error in individual mask sections across the mask section array; and attaching the individual mask sections of the mask section array to a support structure. The support structure allows for expansion and/or contraction of the individual mask sections of the mask section array while maintaining relative positioning of the individual shadow mask section in the shadow mask array. The support structure further allows for compliance of the shadow mask array in a direction normal to the plane of the shadow mask array thereby allowing for a substantially planar interface between the shadow mask array and a substrate to be coated.
The use of smaller mask sections permits the ability to sort out poor quality mask sections prior to assembling the mask sections into a larger mask. This practice results in an overall higher yield of good shadow masks for a given amount of mask material fabricated. Only the mask section is discarded and not the entire shadow mask when a defect is detected. This practice results in further overall cost reduction since only the mask section containing the defect is discarded and replaced. Further,.the likelihood of producing smaller masks without defects is much greater than producing larger masks without defects.
The set or plurality of smaller shadow mask sections are arranged adjacent to each other and bonded to each other thereby forming a mask array. Preferably, the mask sections forming the array are also bonded to a perimetric support structure. Bonding may be performed with an adhesive, or by soldering, brazing or welding.
The total amount of dimensional error in shadow masks that are larger than about 250 mm square can be reduced with the present invention. This is achieved by precisely arranging smaller, more accurate mask sections to form a large mask array. In this way, the accumulation of dimensional error across the entire width and length of the mask array due to mask fabrication variability can be minimized. In addition, the ability to adjust the positioning of each mask section during the assembly process permits compensation for any dimensional error present within the individual mask sections.
In addition to the rigid perimetric support structure or frame, a set of tensioned bands are attached across the opening of a frame. The tensioned bands provide support structure to which individual mask sections may be directly attached in the frame aperture. The tension bands provide the underside mask support needed to prevent excessive mask sag when held in a horizontal orientation while being aligned to the substrate. Unlike the rigid support structure, the tension bands are light in weight and minimize the obstruction of the shadow mask from said vapor deposition source. In a preferred embodiment of the present invention, a predetermined number of tension bands for use with one rigid perimetric support structure or frame are fabricated as a single sheet that will be referred to herein as a band matrix.
Given the nature of this tension band arrangement, mask sections may be mounted to the tension bands at specific attachment point locations that reduce the maximum movement of points on the shadow mask due to a mask temperature increase. In effect, each mask section is constrained in a way to cause thermal expansion to occur about each mask section center instead of expansion about the center of the entire shadow mask. Therefore, in the case where the shadow mask comprises an array of four mask sections, the movement of points on the shadow mask with respect to the mask frame can be reduced by approximately one half.